Thermometer



Jan. 26, 1954 R. c. WAPPNER THERMOMETER Filed Aug. 8, 1951 INVENTORRALPH C WAIZBNER ATTORNEYS atented Jan. 26, 1954 THERMOMETER Ralph C.Wappner, Springfield, Ohio, assignor to The Ohio Thermometer Company,Inc., Springfield, Ohio, a corporation of Ohio Application August 8,1951, Serial No. 240,908 1 Claim. (Cl. 73-353) This invention relates tothermometers particularly adapted for the measurement of temperatures incandy and deep fats.

Such thermometers generally comprise an outer bulb surrounding an innerindicating tube carrying a column of thermally responsive liquid of e.g. mercury or alcohol, and between the lower end of the outer bulb andthe reservoir end of the indicating tube there is positioned a mass ofmetal which serves as a ballast and as a heat conductive medium. Themetallic mass hitherto employed in these thermometers has consisted of afusible metal alloy, for example, an alloy containing 35% by weight oflead and 65% of tin, and the thermometer assemblies constructedtherewith have sufiered from the defects inherent to the usual glass tometal contact.

Thus the normal contraction of the metal in the bulb upon manufactureand repeated heating and cooling of the assembly in service gives riseto poor adherence between the glass and metal, a factor which leads to alooseness of the metal in the bulb and which may result in seriouslylowered heat transmission.

Also in the process of manufacturing the assembly the cooling of themetal about the inner indicator tube exerts a squeezing pressure on thesame leading to a relatively high reject condition.

Further the relatively high specific gravity of the fusible metal alloysunduly increases the weight of present assemblies since the volume ofalloy required must be maintained sufficiently large to permit fusionthereof and to provide a continuous path between the bulb and indicatortube.

It is a primary object of this invention to provide an improvedthermometer assembly which overcomes the above noted defects of presentconstructions.

This and ancillary objects are attained in the structure of invention byutilizing a mass of a fused inorganic salt or a mixture of salts as theballast material to compactly fill the spacing defined between the outerbulb and the reservoir end of the indicator tube.

For thermometers having a maximum operating temperature of less thanabout 400 F. I prefer to use a fused mixture of potassium nitrate andsodium nitrate in the proportions of about 50% by weight of each. Inthese proportions the noted salts have a minimum melting point of about424 F., and accordingly as the salt proportions vary from this ratio themix melting point rises and other weight ratios may be suitably employedto readily provide thermometer assemblies having a maximum temperatureof operation of about 500 F. For example, other salt mixes, such as:

54% potassium nitrate 46% sodium nitrate (melting point 430 F.)

67% sodium nitrate 78% potassium nitrate 22% sodium nitrate (meltingpoint 490 F.)

23 otassium nitrate 77 a: sodium nitrate 33% potass um nitrate (meltingpoint 545 F.) (melting point 450 F.)

Potassium nitrate (melting point 650 F.)

Sodium nitrate (melting point 590 F.) and other salts such as mixturesof sodium chloride and sodium sulphate in varying proportions, arewithin the inventive concept.

The single figure of the drawing illustrates a typical thermometerembodying the present in vention.

In the drawing there is shown at I an elongated tubular outer bulb ofglass in which there is enclosed an indicator tube 2 containing a column3 of mercury or alcohol and a graduated scale 4 with which the indicatortube 2 cooperates. The reservoir end 5 of indicator tube 2 extends intothe closed end portion 6 of outer bulb I. Positioned in the spacingdefined between reservoir 5 and portion 6 is the body I of ballastmaterial which consists of 50% by weight of sodium nitrate and 50%potassium nitrate. This body I adheres closely to the bulb wall portion6 and the inner tube end 5.

In the practice of the invention the salts, that is the potassiumnitrate and sodium nitrate, are thoroughly mixed and then fused tohomogeneity. whereafter the cooled mix is granulated and the requiredquantity of the particles inserted in the closed end portion of theouter bulb. The granulated material is then heated to the fusion point,about 424 F. for equal proportions by weight of the salts, and theindicator tube is thereafter inserted in the molten mass and theassembly allowed to cool. Due to the fact that the binary salt mix hasonly a very slight shrinkage the reservoir end is not subjected to anysubstantial stress, but firm adherence is secured between the fusedsalts and the glass walls of the bulb and indicator due to the inherentproperties of these components, particularly the thermal coefi'icient ofexpansion which approaches that of glass. The thermometer assembly isthereafter completed in the manner known to the art.

Thermometers made in accordance with the above procedure may readilyhave an eifective range between about 285 F. and 500 F.

A primary feature of the invention is the nontoxic character of thesalts utilized permitting ready handling of the molten material withoutthe use of excessive protection equipment in manufacture. Also thisfeature is important for should accidental destruction of thethermometer occur in service a toxicity condition would not be created.

The resultant thermometer assembly has the distinct advantage of highheat transmission to the reservoir of the indicator tube with a loweredthermometer assembly weight and more uniform distribution of the ballastmaterial. The cost of the process is reduced and the product having awhite appearing bulb portion is more attractive to the eye than the darkgray metal of former structures. Among the most important advantages,however, are that the new assembly avoid the looseness of the metal inthe bulb, characteristic of former structures, and has the furtheradvantage of lowered manufacturing rejects due to the substantialelimination of the squeeze pressure on the indicator tube duringmanufacture.

'It will be understood that this invention is susceptible tomodification in order to adopt it to 4 different usages and conditionsand, accordingly, it is desired to comprehend such modifications withinthis invention as may fall within the scope of the appended claim.

I claim:

A thermometer comprising an outer glass bulb having a closed lower endportion, an indicator tube enclosed by said bulb and having thereservoir end thereof extending into the said closed lower end portionof said bulb to define a spacing therebetween, and an integral bodycomprising a homogeneous mixture of fused inorganic salts compactlyfilling the said spacing between the said reservoir end and lower bulbportion and firmly adhered thereto.

RALPH C. WAPPNEPU.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,316,780 Freas Sept. 23, 1919 2,051,540 Chaney Aug. 18, 19362,271,193 Hohmann Jan. 27, 1942

